We review the first six years of radio observations of Supernova 1987A. The evolution can be divided into two phases: the initial radio outburst which lasted a few weeks, and the period from mid-1990 to the present, during which the radio emission has steadily increased. Both phases can be explained by a small fraction (0.1–0.5%) of the post-shock thermal energy being converted to energy in relativistic particles and magnetic fields, which give rise to synchrotron radiation. The optical depths, densities and density profiles for the pre-shocked circumstellar material are somewhat different for the two phases, but consistent with models of the density structure of the material within the circumstellar ring. New high-resolution radio observations show that the SN shock front is already at about three-quarters of the radius of the circumstellar ring, and that there exists a bright equatorial component of emission aligned with this ring which is probably due to a polar density gradient in the ‘hourglass’ structure.

Introduction

Radio studies of supernovae began with the detection of SN 1970G in M101 (Gottesman et al. 1972; Allen et al. 1976), though it wasn't for another decade that detailed radio light curves were available for a statistically useful sample of supernovae. Mainly through the work of Weiler, Sramek and collaborators (this volume) at the Very Large Array, there are now over a dozen well-studied examples of radio supernovae (RSN).